Which statement is true?

• A. Renal tubular cells originate from the renal pelvis.
• B. Red blood cells in acid urine (pH 4.5) will usually be crenated because of the acidity.
• C. Bacteria introduced into a urine specimen at the time of the collection will have no immediate effect on the level of nitrite in the specimen.
• D. Pilocarpine iontophoresis is the method of choice for the collection of pericardial fluid.

Answer: (C)

The key idea is how nitrite appears in a urine test. Nitrite in urine comes from certain bacteria that convert nitrates (from the diet) into nitrites. This transformation requires time for the bacteria to act, so nitrite may be negative if the urine is tested soon after collection or if no nitrate-reducing bacteria are present. If bacteria are merely introduced into a urine sample at the moment of collection, there isn’t enough time for those bacteria to convert a noticeable amount of nitrates to nitrites before testing, so the nitrite level remains unchanged in that immediate moment. Only after incubation of the sample for several hours would nitrite be expected to rise if nitrate-reducing bacteria are present.

Renal tubular cells do not originate from the renal pelvis; they arise from the metanephric mesenchyme, while the pelvis and calyces are lined by transitional epithelium derived from the ureteric bud. Red blood cells in urine are not reliably crenated simply because the urine is acidic; crenation is a response to hypertonic solutions, and urine’s acid-base status alone doesn’t determine this morphology. Pilocarpine iontophoresis is used to stimulate sweat production for sweat chloride testing, not to collect pericardial fluid; pericardial fluid is obtained via pericardiocentesis.